The development of mid-infrared (IR) waveguides has been driven by their use as remote or small-sample-size chemical sensors for surface sensitive spectroscopy. Such waveguides can be thought of as miniaturized multiple reflection elements (MREs) wherein the incident light undergoes total internal reflection at the interface between media of different refractive indices. At each internal reflection within the waveguide, a portion of the optical field, the evanescent wave, extends beyond the high-index waveguide into the adjacent low-index medium, to a depth (dp) dependent on the angle of incidence and the ratio of the two refractive indices. The ability of molecules outside the high-index waveguide, but near its surface, to absorb energy travelling through the waveguide via this evanescent wave makes possible the phenomenon known as attenuated total reflection (ATR) or evanescent-wave spectroscopy (EWS).
In the IR region, high-refractive-index materials as Ge, Si, and KRS-5 (Tl2Brl), cut and polished as prisms having trapezoidal or parallelogram cross-sections and dimensions on the order of 50xc3x9720xc3x972 mm, are in common use for EWS measurements. These macroscopic waveguides typically have throughputs matched to commercial FTIR spectrometers, i.e. in the vicinity of 1-10 mm2-stearadian. Commercially available IR fiber optics (multimode cylindrical waveguides made of, e.g., chalcogenide glass), have more recently been used as EWS sensors. These optical fibers typically have much lower throughputs than the prism MREs, complicating somewhat their use with commercial IR spectrometers. Nevertheless, when properly coupled to a small-area (low-noise) IR detector, fiber optics display the advantage that miniaturization enables smaller amounts (xcexcL) of sample to be detected. This advantage arises from the fact that, while the surface sensing area is smaller, the light experiences a larger number of reflection per unit length of waveguide, yielding a concomitant increase in evanescent path length. It would be desirable to see how far this advantage could be extended, i.e. how thin an EWS waveguide or fiber could be made. However, it becomes impractical to make a free-standing IR fiber less than xcx9c50 xcexcm in diameter.
Most thin planar waveguide development has been in the visible region, where low-loss transparent materials (polymers and glasses) are commercially available and easy to manipulate. Such waveguides have generally been used in conjunction with single-frequency lasers, which provide high luminosity, monochromaticity, and fine control over the launch angle, and have been used for absorption, Raman, and fluorescence analytical methods. In contrast, IR-transmissive materials with the requisite high refractive indices and low attenuation values are either very brittle, or have not had techniques developed to allow them to be deposited (e.g. by evaporation or sputtering techniques) as uniform and well-adhered films of the desired thicknesses of 1-100 xcexcm.
The disclosed supported planar and tapered, quasi-planar waveguides provide increased broadband transmission and demonstrate many of the characteristics predicted by planar waveguide theory. In particular, they show a great increase in the sampling sensitivity as compared to previous evanescent-wave absorption measurements.
There are three particularly novel aspects to the fabrication and use of the disclosed thin supported planar IR waveguides. First, the waveguides have been generated by physically xe2x80x9cwhittling awayxe2x80x9d at a macroscopic piece of highly transparent single-crystal materials, such as Ge., rather than by attempting either to deposit or to modify chemically a thin film of transmissive material. The latter are the most common approaches for generating thin-film waveguides. For example, sputtering is the only method to have been used previously in an attempt to fabricate thin-film Ge light guides for wavelengths in the 2-10 xcexcm range. However, this attempt results in a waveguide with rather high attenuation of about 20 dB per cm, due to scattering from the non-uniformly-deposited Ge. It is possible to detect transmission of CO2 laser light through such a waveguide, however attempts at detecting broadband transmission through similarly-fabricated thin-film-sputtered waveguides, e.g. 1-xcexcm thick Ge on CaF2, have failed. This is likely due to the much lower luminosity of the broadband light source available, as compared to the CO2 laser used in prior art testings. The disclosed devices enable success in obtaining IR transmission using the weaker broadband source through the development of waveguides with much lower scattering losses than currently are obtainable with sputtered Ge films.
The disclosed waveguides further have an added xe2x80x9ccladdingxe2x80x9d for the waveguide""s supported surface, in the form of a rather thick vacuum-deposited layer of a cladding material such as ZnS. This turns out to be crucial for fabrication and use, since it is difficult to attach the piece of bulk single-crystal Ge to a substrate without using IR-absorbing adhesive materials. Only by protecting the Ge with the vacuum-deposited cladding is it possible to use simple cements or optical adhesives to attach it firmly enough to allow grinding and polishing to a few-xcexcm thickness.
The disclosed waveguide further uses a direct method to couple light into and out of its ends. Such direct coupling is generally not used for monochromatic (e.g. laser) light; more efficient coupling methods exist (e.g. prism coupling) that depend on optical interference effects. However, use of curved mirrors with foci at the two ends of a waveguide is probably the most generally useful means of coupling a broad bandwidth of light into it. This has long been known to be true for macroscopic MREs used for EWS. This is also be true for waveguides of arbitrarily thin dimension, although in some thickness ranges, the waveguides show considerable oscillations of throughput, as disclosed below.
Supported planar IR Ge waveguides, having a thickness between 50-100 xcexcm, are useful as mid-IR evanescent-wave sensors. A significant portion of the light energy transmitted through such waveguides actually propagates outside the germanium, as an evanescent wave in the surrounding medium. With  less than 100-xcexcm-thick waveguides, a very small number of IR-absorbing molecules near the surface of the waveguide can significantly attenuate the light transmitted through the waveguide, allowing the measurement of an ATR (attenuated total reflection) spectrum. Sizable ATR bands are therefore observed from thin surface layers under 1 mm2 in area. This includes thin coatings on small pieces of polymer film, as well as patches of the plasma membrane of large individual cells, e.g. frog oocytes.
One difficulty with using thin planar Ge waveguides as internal reflection elements (IREs) is coupling measurable amounts of light through such waveguides and onto a detector. Prior art requires the use of an IR microscope in order to measure useful spectra through waveguides having a thickness between 30-100 xcexcm. Use of a microscope, however, results in significant limitations on the waveguide configurations that can be used. In particular, waveguide lengths were generally limited to xcx9c12 mm, the maximum separation between objective and condenser focal points on commercial FTIR microscopes. Furthermore, the waveguides had to be positioned vertically, i.e. along the optical axis of the microscope. This is an inconvenience for samples containing liquids, especially small biological samples.
A quasi-planar waveguide, preferable made from single-crystal germanium, is also disclosed wherein one of the surfaces has an arcuate contour while a parallel, second surface is planar, the first surface being concave relative to the second surface. The perimeter is comprised of multiple opposing planar surfaces at right angles to the second surface. The second surface is coated with a cladding, such as ZnS and then adhered to a substrate, such as quartz. The substrate must have a perimeter at least equal to that of the waveguide and a thickness sufficient to support the waveguide. The arcuate surface of the waveguide has an apex at least about four times greater than the nadir, with a preferred ratio of nadir to apex taper of at least about 1:10 and up to about 1:50. For clarity and consistency, the ratio can also be reviewed from the reverse standpoint, that is, apex to nadir. Thus, the ratio of apex to nadir is up to 0.25:1. The preferred ratio of nadir to apex is in the range from about 0.01:1 to about 0.25:1. The nadir of the waveguide is less than 100 m, and preferably in the range of 1 to 20 m. The arcuate surface is polished to about a 0.1 m finish to prevent light scattering. The tapered waveguide can also be coupled directly to an IR detector, eliminating the need for a microscope and enabling more accurate alignment. The elimination of the microscope also enables the waveguide to be mounted horizontally. The tapered waveguide increases IR signal throughput by about 4-5 fold, a result of filling the large numerical aperture of a high-index waveguide medium (Ge, n=4). This increase, for a given sensor thickness, makes it possible to detect the IR signal level more precisely in a shorter length of time. With an untapered planar waveguide, the largest numerical aperture that can be attained inside the waveguide is equal to the numerical aperture of the element that focuses light through air onto the end of the waveguide. This must always be less than 1, and for commercially available focusing optics is typically 0.5-0.8. On the other hand, the fundamental limitation on the largest numerical aperture that can be propagated inside a dielectric waveguide is the refractive index of the waveguide material and it""s cladding, and is equal to (n12xe2x88x92n22)xc2xd. Here n1 is the refractive index of the waveguide medium (n1=4 for Ge), while n2 is the highest refractive index of the cladding materials in contact with the waveguide (n2=2.26 for ZnS). For the disclosed ZnS-clad Ge waveguide, this maximum numerical aperture is 3.3, or approximately 4-fold higher than the numerical aperture of available focusing optics. In theory, at least xcx9c4-fold more light energy can be propagated through the sensing region of a planar Ge waveguide than can be obtained by focusing light through air into the edge of an untapered waveguide of the same minimum thickness. This theoretical maximum throughput is, in fact, closely approached with the tapered waveguide design